This invention relates to novel compositions of matter and more particularly to doped titanium oxide additives exhibiting high electrical conductivity.
Battery manufacturers are continually trying to improve the electrochemical performance of their batteries, since even relatively small improvements of performance characteristics such as discharge capacity, cycle life and shelf life can provide advantages in a highly competitive market. At the same time, the current requirements of electronic devices that are powered by batteries are increasing, and the capacity at higher discharge rates is becoming a more important characteristic for these batteries. This is particularly true of both primary and rechargeable alkaline batteries.
One approach that has been used to improve the discharge capacity and/or cycle life of electrochemical cells is to use performance enhancing additives in the negative electrode, positive electrode and/or electrolyte. Such additives may affect these improvements by reducing the internal resistance of the cells, increasing ionic conductivity, preventing the formation of undesirable byproducts, and so forth.
Evans (U.S. Pat. No. 4,465,747) discloses a nonaqueous cell with an organic solvent electrolyte and a cathode containing MnO2 and a minor amount of an additive selected from the group consisting of borates, silicates, molybdates, phosphates, aluminates, niobates, tantalates, titanates, vanadates, zirconates, manganates, cobaltates and tungstenates of alkali metals or alkaline earth metals. Langan (U.S. Pat. No. 4,478,921) discloses a nonaqueous cell with a cathode containing MnO2 and a minor amount of MnCO3. The purpose of these additives is to minimize the increase in internal impedance of the cell during storage or discharge. The additive is believed to prevent degradation of the electrolyte caused by reaction of the electrolyte with surface acidic groups on the MnO2 or incompletely neutralized salts used in preparing the electrolyte solution.
Taucher et al. (PCT Patent Publication No. WO 93/12551) disclose primary and rechargeable alkaline Zn/MnO2 cells with barium compounds (e.g., oxide, hydroxide or sulfate) added to the cathode to improve cell capacity. The barium compound slows down the formation of hetaerolite, which cannot participate in the cycling process and tends to expand and destroy the cathode structure. Cathode additives for improving the capacity of alkaline Zn/MnO2 cells have been disclosed. Swierbut et al. disclose the use of one or more of SnO2, Fe2O3xe2x80x94TiO2, TiO2 (P-25), BaTiO3, K2TiO3, Nb2O5 and SnO in U.S. Pat. No. 5,599,644. Nardi et al. disclose the use of at least one of SrTiO3 or CoTiO3 in U.S. Pat. No. 5,895,734. CaWO4, MgTiO3, BaTiO3, CaTiO3, ZnMn2O4 and Bi12TiO20 are disclosed by Davis et al. U.S. Pat. No. 5,532,085. Mieczkowska et al. disclose the use of Bi2O3, PbO2, SnO2, Co3O4, CoO, Bi2O3.3ZrO3 and K2Cr2O7 in U.S. Pat. No. 5,516,604. In U.S. Pat. No. 5,342,712, Mieczkowska et al. teach that the useful service life of primary alkaline Zn/MnO2 cells can be extended as a result of increased mobility of ionic flow during discharge by adding anatase TiO2 to the cathode.
While there may be advantages to using such additives, in many cases the advantages can be at least partially offset, particularly on heavy drains, by the relatively high resistivities of the additives which increase the internal resistance of the cell.
It is known that the physical and electrochemical properties of semiconductor materials such as SiO2, TiO2 and SnO2 can be modified by doping those materials with other cations. This approach has been used to modify materials for use in the fields of semiconductors, photoelectrochemistry and solid state sensors. For example, Unexamined Japanese Patent Publication No. 10-316,429 discloses that anatase TiO2 is suitable for use as a white electroconductive powder in the manufacture of semiconductors if it is doped with zinc or aluminum and its surface is coated with an electroconductive film of metal oxide. Karakitsou et al. disclose the use of cation doped TiO2 exhibiting improved performance as a photocatalyst in water cleavage in the Journal of Physical Chemistry, 1993, vol. 97, pages 1184-1189. Katayama et al. disclose the use of Nb2O5-doped TiO2 ceramics exhibiting improved humidity sensitivity.
Traditional TiO2 additives (anatase or rutile) for alkaline batteries are insulating materials that reduce the overall conductivity of the cathode. Conductive forms of titanium oxide exist in the family of Magneli phase substoichiometric titanium oxides with the formula TinO2nxe2x88x921 where n is from 4 to 10. The Magneli phases exhibit different crystal structures than rutile TiO2. WO 97/27344 and EP 0 572 559 disclose that the Magneli phases are useful in electrochemical applications, such as in electrodes of an electrochemical cell.
However, there remains a need for performance enhancing additives for electrochemical cells that provide improved discharge capacity, especially on high current drains, and which provide rechargeable electrochemical battery cells exhibiting improved cycle life.
One aspect of the invention is a novel compound represented by the formula:
Ti1xe2x88x92xMxO2xe2x88x92y
where M is an element having an octagonal coordination structure, x is from about 0.01 to about 0.5, and y is from about 0.05 to about 0.25. These compounds exhibit significantly reduced electrical resistivity as compared with conventional titanium dioxides. The compounds are useful performance enhancing additives for electrochemical battery cells. For example, the novel compounds of this invention may be used in electrochemical battery cells, both primary and secondary (rechargeable), to improve discharge capacity.
Another aspect of the invention is a titanium oxide compound having an expanded rutile crystal structure and a resistivity less than 100 ohm cm.
A further aspect of the invention is a process for making a compound represented by the formula:
Ti1xe2x88x92xMxO2xe2x88x92y
where M is an element having an octagonal coordination structure, x is from about 0.01 to about 0.5, and y is from about 0.05 to about 0.25. The process generally comprises mixing titanium dioxide with an oxide of M; heating the mixture for a time and at a temperature that are sufficient to cause reaction of the titanium oxide with the oxide of M to form a doped titanium oxide, and subjecting the doped titanium oxide to a reducing atmosphere to remove oxygen atoms from the doped titanium oxide.
In another aspect of the invention, a titanium oxide compound having an expanded rutile structure and a resistivity less than 100 ohm-cm is prepared by a process comprising mixing titanium dioxide with an oxide of M, heating the mixture for a time and at a temperature that are sufficient to cause reaction of the titanium dioxide with the oxide of M to form a doped titanium oxide, and subjecting the doped titanium oxide to a reducing atmosphere to remove oxygen atoms from the doped titanium oxide.
Other aspects of the invention involve the use of the compounds described above in electrodes and electrochemical battery cells.
These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings.